Magnetometer



arch 9, E954 L. L. BURNS, JR 2,671,275

j f MAGNETOMETER Filed March 31, 1949 2 Sheets-Sheet 1 farch 9, 1954 L. BURNS, JR

MAGNETOMETER 2' sheet-sheet 2 Filed March 31, 1949 Hillllllwll'lclllullllllllll Illll INVENTOR 155m; Eme/y' Je ATTORNEY Patented Mar. 9, 1954 MAGNETOMETER Leslie L. Burns, Jr., Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application March 31, 1949, Serial No. 84,621

' 2 Claims.

My invention relates to improvements in apparatus for measuring magnetic elds, and particularfy to apparatus for measuring the eld strength, or the direction, or both, of a magnetic eld. For simplicity, an apparatus of this kind will be referred to herein and in the appended claims as a magnetometer.

Magnetometers have been proposed which depend for their operation on the motive effect of a magnetic ield on a current-carrying.conductor (see e. g. U. S. P. 1,863,415), on the reaction between a current-carrying conductor and the eld to be measured (see e. g. U. S. P. 1,886,336), or on a measurement of the current required to saturate a permeable core located in the eld to be measured (see e. g. U. S. P. 2,047,609). These and other comparable types of magnetometers either require accurately mounted moving parts or involve complicated apparatus with a plurality of coil structures and the like.

It is a principal object of my invention to provide an improved magnetometer which does not depend for its operation on relative motion between the parts thereof, and which is characterized by extreme simplicity and sensitivity.

Another object of my invention is to provide an improved apparatus for measuring and indicating the relative direction of the earths magnetic eld.

In accordance with my invention, the foregoing and other objects and advantages are attained by locating a core member of variable permeability (defined hereinafter) in the magnetic circuit of an inductor, in inductance-affecting proximity thereto, and measuring the effect produced on the inductance of the inductor by permeability changes occurring in the core member when the latter is placed in the eld to be 'measured For simplicity, the term core member is used herein and in the appended claims to mean a magnetically permeable member located in inductance-aecting proximity to an inductor, and is intended to include the case of a permeable member surrounded by an inductor, or a permeable member in the form of a sleeve surrounding an inductor, as well as other comparable arrangements wherein an appreciable portion of the flux linkages forming the magnetic eld of an inductor will pass through the permeable member so that the effective inductance of the in- 'ductor will be a function of the permeability of the core member. The permeability of a core member will be understood to mean the readiness with which the core will accept magnetic ux, or, more precisely, the ratio between the amount of magnetic flux passing through the core and the magnetizing force producing the ux. Where a material is referred to herein and in the appended claims as having variable permeability, it is intended to mean that the magnetic permeability of material so referred to varies as a function of the magnetic ux density therein.

A more complete understanding of my invention can be had by reference to the following description of illustrative embodiments thereof, when considered in connection with the accompanying drawings, wherein:

Fig. 1 is a schematic diagram of a magnetometer arranged in accordance with my invention;

Fig. 2 is a block diagram of a magnetic compass embodying the principles of my invention;

Fig. 3 is a graph illustrating one of the operational characteristics of the apparatus illustrated in Fig. 2;

Fig. 4 is a schematic diagram containing specic illustrative circuits corresponding to the blocks in Fig. 2; and

Fig. 5 shows a modied form of indicator suitable for use in the apparatus of Fig. 2.

Referring to Fig. 1 of the drawing, illustrating one embodiment of my invention, there is shown an inductor in the form of a, coil l0, wound on a variably permeable core member I2, together with apparatus for measuring the inductance of the coil l0. The core member l2 preferably consists of so-called ferrite material (i. e. a homogeneous crystalline compound comprising the reaction product of iron oxide and at least one other metallic oxide), although other materials can also be used which possess the property of variable permeability, as that term has previously been defined herein.

When the coil Ill and the core i2 are placed in a magnetic eld I4, the flux in the eld I4 will pass through the core I2, causing a decrease in the permeability thereof. Since the inductance of an inductor varies directly as the permeability of the magnetic circuit path for the inductor (i. e. the path that will be followed by a magnetic field generated by current iiow through the inductor), any increase or decrease in the permeability of the core l2 will result in a corresponding increase or decrease, respectively, in the effective inductance of the coil I0. Accordingly, the effective inductance of the coil I0 will be decreased by the magnetic flux in the core l2 due to the field Ill, and measurements of the inductance of the coil l0 can be utilized as measurements of the field I4.

While any desired method can be used for measuring the inductance of the coil l0, a preferred method is to connect the coil to a signal generating network to control the frequency of the signals generated therein, and to measure the frequency of the signals generated in the network as a measure of the inductance of the coil.

In the apparatus shown in Fig. l, the coil lli is connected in circuit with a triode tube I6 through a shielded cable 20. The cable 20 allows the coil I and the core l2 to be moved about at will in measuring magnetic fields, while the stray capacitance 2l between the shield 22 of the cable 20 and the shielded leads 24 is effectively in parallel with the coil I0, thereby forming the resonant circuit of a so-called Colpitts oscillator.` A vari; able capacitor I8 is connected in parallel with the resonant circuit of the oscillator to permit frequency adjustment for calibration purposes and the like.

In order to measure the frequency of the signals from the oscillator tube I6 accurately, it is deemed preferable to mix the signals from the oscillator tube I6 with those from a stable, fixed frequency oscillator, such as a crystal oscillator 26, and to measure the difference in frequency between the two signals. To this end, the output lead 28 from the oscillator tube I6, and the' output lead 30 fromthe crystal oscillator 26, areoonnected toa' pentag'rid mixer tube 32, wherein the two input signals are" combined to obtain sum and difference frequency components in the output from the mixer 32. A filter circuit 34 serves to eliminate all compbne'nts of the' signals from the mixer 32 except those corresponding to the difference in frequency between the two input signals,- and a frequency meter 36 is provided for measuring the frequency of the desired differencefrequeney component. `The meter 36 can be calibrated in terms of magnetic field strength, or in any convenient units for the particular use involved.

If it is' desired to determine the direction of a magnetic field with the apparatus of Fig. 1, a small unidirectional field in the ocre member l2 can be used to provide directional sensing.

For example, a battery 38 can be connected in 'iSL aligned ,with @afield being measured, the 'field due to' the current from the battery 38 willseither add to or subtract from the field being measured (depending on whether the two fields are running in the same or opposite directions),

thereby increasing or decreasing the frequency of the signals from the oscillator tube I'B. The exact direction and strength of the unknown field can be ascertained by manipulating the coil l0 and the core l2 while observing' the indications of' the meter 36. A s s y,

A magnetometer of the type shown `in Fig. l is especially useful as a laboratory instrument, as it has beenfound that magnetic fields of the order of .025 gauss can be measured readily with an instrument of the foregoing type. The principles of my invention Afind furtherqapplication in apparatus for measuring the earths magnetic field, or a so-called magnetic compass, and Fig. 2 of the drawing there is shown, in block diagram form, a magnetic compass arranged in accordance with the invention.

In the apparatus of Fig. 2, two indctance coils 50, 52 are located at right angles toeach other,

and provided'with core members 54, 56 of variable Cal permeability. Two batteries 58, 60 are provided for supplying a small current to the two coils 50, 52, in order to establish a small magnetic field through the cores 54, 56. For convenience of discussion, the fields due to the currents from the batteries 58, 60 are assumed to be in the direction of the arrows A, B, respectively. Taken togetherrthe two coilland-ccre members 50, 54, and 52, 56, c'ornprise a c'r'nposite coiland-core structure C. For convenience, the directions indicated by the arrows A and B will be referred to herein as the primary and secondary directional axis Y of the structure C, respectively, although i t will be appreciated that the two axes A, B, have no inherent features of distinction other than the arbitrary designations assigned to each.

Assuming that the structure C is located in the earths magnetic field, with the primary directional axis A parallel to, and in the same direction as. the lines of the earths field, then the fiux in the earths field will add to the flux due to the current from the battery 5|), the magnetic flux density in the core 54 will be at a maximum, and the effective inductance of the coil 50 will be at a minimum, in accordance with the relations between inductance, permeability,

and fiux density previously mentioned. At the same time, the earths field will be at right angles to the secondary directional axis B, 'andwill neither add to nor oppose the constant eld through the core" 56 vdue to current from the battery 6U. Consequently, the effective vin- "ductance of the coil 52 will have some value intermediate between a maxir'riumand a minimum value. Assuming, next, that the structure C is rotated in a counter-clockwise'direction (in the plane of the earths magnetic field), the flux density in the core 54 will decrease during' the rst 180 degrees of rotation of the structure C, and will increase during the second 180 degrees of rotation. At Vthe same time, the flux density in the core 56 will increase during the first and last degrees of 'rotation of the structureC, and will decrease during the intermediate' 180 degrees of rotation. Consequently, the effective inductance of the coil .50 will increase to a maximum, and then decrease to a minimum, vduring the rotation of the structure C, while the effective inductance of the coil 52 will first decrease, then increase, and then decrease again toits original intermediate value at the end of 360 degrees of rotation.

The foregoing changes inthe effective inductance of the coils 50, 52 (during 360 degrees of rotation of the coil-and-core structure C) are diagramr'ned in Fig. 3, wherein the effective in- ,ductance of the coils 50, 52 is plotted as the imagneticfield, (thereis, a corresponding pair of inductance values for the coils 50, 52, and lthat no two Ypositions of the structure C will give the same combination of values of inductance for the coils 50, 52. Accordingly, a concurrent indication of the effective inductances of the coils 56, 52 will also be an indication of the direction ofthe earths eld with respect to the difrectional axes A, B yof the Yc'ore-and-coil structure C.

One arrangement of apparatus for obtaining an indication of the relative inductancs of the coils 50, 52 is shown in block diagram form in Fig. l.

Fig. 2, and `speciiic circuits and indicators, illustrative of apparatus corresponding to the blocks in Fig. 2, are shown in Figs. 4 and 5.

In the block diagram of Fig. 2, each of the coilsl 50 and 52, is connected to a signal generating network, B2 and 64, respectively. The frequency of the signals generated in the networks 62 and 64 is controlled by the effective inductance of the coils 50 and 52, respectively. The signals from each of the networks 02, 64 are supplied to frequency discriminator networks 66, 68, each of which will generate a voltage representative in polarity and magnitude of the frequency of the signals supplied thereto. Both of the discriminators 66, 68 are connected to an indicator which will provide a composite measurement and indication of the relative polarity and magnitude of the voltages from the discriminators 66, 68, and, hence, of the frequency of the signals from the signal generating networks 62, 643.

Since the frequency of the signals from the networks 62, 64 will be a function of the direction of the earths magnetic field relative to the directional axes A, B of the coil-and-core structure C, the indication produced on the indicator 10 will be a frequency measurement representative of the direction of the earths eld. In Fig. 4 there are shown specific circuits illustrative of those indicated by blocks in Fig. 2. For simplicity, the circuits for only one of the coils, 50, are shown in Fig. 4, it being understood that similar group of circuits is indicated by the dotand-dash outline associated with the coil 52.

In Fig. 4, the coil 50 is connected to a signal generating network 62 which includes a pentagrid tube 16. Two of the grids, 18 and 88, in the tube 16 serve as the control grid and anodef respectively, in a Colpitts oscillator network, the resonant circuit for which comprises the coil 50 and two capacitors 12, 14. rlhe tube 16 serves both as an oscillator tube (in conjunction with the circuit elements 50, 12, 14), and as a mixer tube, wherein the signals generated in the oscillator portion of the tube 1t are mixed with signale of higher frequency from a fixed frequency L signal generator 26, similar to the network in The output of the tube 16 is connected to a discriminator network 66, of a type well known in the art. The resonant circuits 84, 88 in `the discriminator 06 are both tuned to a fre- 'the primary axis of the structure C is at Yright angles to the earths magnetic field. The

discriminator network 66 will generate no out- -put voltage when the difference frequency 0f the signals from the tube 16 is equal te the center frequency of the discriminatcr. and :will generate a positive or a negative voltage when the difference frequency of the signals from the tube 16 is above or below the center Afrequency. Thus, for example, if the primary axis A is aligned with the earths field. with the latter adding to the xed iield due to the battery 58,

the frequency of the signals generated in the oscillator section of the tube 16 will be a minimum,

the difference frequency will be a maximum, and the output voltage from the discriminator network B6 will have a maximum positive value. If -the earths field and the primary axis A are laligned but extend in opposite directions, the

'6 discriminator output will be a maximum negative voltage. Intermediate angular relations between the primary axis A and the earths field will produce intermediate voltage outputs from the discriminator 56.

It should be noted that the signal generating network 62 in Fig. 4 could comprise merely an oscillator corresponding to the oscillator portion of the tube 16, without the xed frequency oscillator 26, and without the remaining portion of the tube 10. However, it is preferable to tune the oscillator which is directly controlled by the coil 50 to the highest suitable frequency which is convenient, in order to obtain the maximum possible frequency change with changes in effective inductances of the coil 50. On the other hand, the discriminator network 06 will be more sensitive to small frequency changes about a low center frequency, and it is, therefore, deemed preferable to mix the signal from the oscillator section of the tube 16 with a xed frequency signal to obtain a relatively low frequency resultant, say of the order of 500 kilocycles.

The output of the discriminator network 66 is connected to the grid of a D. C. amplier tube 88 in an indicator control network 61, which supplies one set of deflection voltages to a cathode ray tube indicator 10a. The control network 61 includes a sawtooth signal generator, which consists of two gas triode tubes 90, 98, connected in parallel with a capacitor 93, and resistors |06| |4. The anode 90 of one gas tube 90 and the cathode |00 of the other gas tube 98 are connected to the voltage supply source (not shown) through two of the resistors, |06, |08, one of which |08) serves as the anode load resistor for the D. C. amplifier tube 88. The cathode 92 of one gas tube 90 and the anode |04 of the other gas tube 98 are connected to the voltage supply source through the resistor H0 and through one of the resistors H2 in a voltage divider H2, |4. A biasing voltage source I0 is provided for the D. C. amplifier 88, and the magnitude of the voltage from the bias source H6 and the resistance of the resistors U16-H4 are so selected that the voltage at the anode of the D. C. amplifier 88 will be equal to the voltage at the junction point H5 of the resistors H0 and H2 when the output of the discriminator 66 is zerc- (i. e. when the difference frequency of the signals from the tube 16 corresponds to the center frequency for the discriminator 06). If the difference frequency of the signals from the tube 16 inthe signal generator 62 is above the center frequency, the discriminator will-supply a positive voltage to the amplifier 88. causing the voltage at the anode of the amplifier to become less positive than the Voltage at the junction H5 of the resistors H0, H2. A decrease in the difference frequency of the signals from the tube 16 will have the opposite effect, causing the voltage at the anode of the amplifier tube 88 to become more positive than the voltage at the junction H5 of the resistors H0, H2,

The grids 94, |02 of the gas tubes 92, 98 are both connected to a trigger pulse generator H8, consisting of a gas triode |20, a capacitor |22 connected in parallel therewith, a charging resistor |24, and. a cathode resistor |20 for the tube |20. The capacitor |22 in the trigger generator H8 will alternately charge through the resistor |24 and discharge through the gas tube |26 and the cathode resistor |26, at a constant `repetition rate, generating positive pulses of 7 'voltages .across lthe resistor `.|26, and, hence, :at :the grids .'94, |92 ,of the gas tubes .90, `9,8.

LIn .the control network 67|, if the diiference frequency of .the `signal `from the tube 15 in the signal generating network 62 is .above the .center frequency of v.the discriminatonso `that the voltage `at ,the anodeof the tube is moreigiositive than :the voltage .at .the junction `|l| of the `resistors H0, |2, then charging current will flow .to-.the capacitor 93 through the resistor |-.06,.and the capacitor93 `will .discharge through xthe gas tube 92 onithe occurrence of `each pulse from fthe .trigger pulse .generator ||8, thus generating a "negative-ssawtooth wave at'the anode 96 ofthe tube 90, and a -positive sawtooth vWave at the cathode .92 of .the tube 90. If 'the difference frequencyof the signals from the tube 19 is below the center frequency of vthe .discriminator '66, ithe vpolarity of thevoltages at the anode of the .tube

v38 and at the junction ||5xof the Aresistors ||0, f-

||2 will reverse, bringing the other ygas tube 98 into operation, and `causing the capacitor 93 Ato charge inthe opposite direction, thus reversing the polarity of the sawtooth waves on the output leads |28 from the indicator control network 67.

The leads |28 from'the control network 61 are connected to one of the sets of deflecting plates |530 of the cathode ray tube indicator 10a., while corresponding leads |29 from the control network A69 (associated with the coil 52) are connected to the other set of `deilecting plates |3| inthe cathode ray tube "a,

Assuming that Athe cathode ray beam in the tube 10a is initially centered on the screen |32,

the sawtooth voltage waves from the network 5l will repeatedly tend yto deflect kthe beam either upwardly or downwardly Afrom the center of the screen, a greater or less amount, depending on the polarities and magnitudes of the sawtooth waves from the network `6l as determined by the angular relation between .the primary axis A of the structure C and the earths magnetic field. At the same time, the sawtooth voltage waves from the control network 69 will repeatedly tend to deflect the cathode ray `beam a greater or less distance to the right or to the left of the center -of the screen |32, depending on the angular lrelation between the secondary axis B andthe earths'magnetic field. As a result, the cathode raybeam will be deflected repeatedly along a ,A

line determined by the vector sum of the sawtcoth voltages from the control networks 51, B9. The screen `|32 of the cathode ray tubela is provided with a peripheral scale corresponding .to the `points of the compass, and the tube is oriented so that when the primary axis A of the structure C is aligned in the same direction as the earth's magnetic eld, the line `traced by the cathodenray beam on the screen |32 will point toward Zerodegrees (north) on the peripheral scale.

The coil-and-core structure C can be permanently mounted in an airplane, boat, vor the like, and the mounting means may include gimbals or similar means lto insure that the plane of thecoiland-core structure C will remain parallel with the plane of the earths field regardless of the attitude assumed by the craft in which the Yapparatus is mounted. Such structural details are not shown since they are well known in the art and form no part of the present invention per se.

In Fig. 5 there is shown an alternative form of indicator 'lilb suitable for use in an apparatus of the type shown -in Fig. 2.

The indicator 10b shown in Fig. 5 is an 'electrovmagnetic .device comprising .a 'pa-1r of coils |540, |42, mounted :at .right angles .toeach oja-bemand having icore members .144,Y |1415 associated therewith. The .core members |44, |46 are preferably of substantially unvarying permeability so that the indicator 10b -will be relatively unaffected by extraneous magnetic fields which may pass through the cores |44, |45. The indicator 10b also includes a magnetized needle |48 whichis pivotally lmounted vat the intersection O f the crossed coils |40, |42, -so as to be subject :to :the combined effect of magnetic fields Ageneratedby current in V:the :coils |40, |42.

The coils |40, |42 .are supplied with unidirectionalcurrents from two D. C, amplifiers |50. 1115.2,

only one of which isshown 'in detail. The D. C. amplifiers |50, |52 each receive control voltages from two :discriminators 66, 68, and (the discrimj.- nators 165, 68 are each connected to signal generating networks (not shown), as in the apparatus of Figs. 2 and 4. Accordingly, the output voltage from each ofthe D. C. amplifiers |50, ,|52 will 4be a function of the frequency of the Signals from thesignal generating networks (not shown), which, in Vturn will have their frequency controlled bythe relative orientation of the earths eld and the core-and-coil structure C as in Fig. ,4. r'hemagnitude of :the current through .thegindicatorcoils |49. |42 in Fig.-5 will be dependent oni-.he frequency of the signals .received by lthe vdiscriminators S6, 58, andthe vector sum of the magnetic fields ofthe coils |40, |42 will determine the relative position of the indicating needle |48. In view of the explanation already given ofthe loperation of the apparatus ofFig. 4itis believed lthat a detailed discussion of the operation of the apparatus-shown in Fig. 5 is unnecessary.

Since the apparatus illustrated in Figs. 2, 4, and 5 involvesignals which are suitable for radio transmission, it is apparent that a magnetic compass embodying my invention is suitable for telemetering applications, as well as for self-contained instruments. For example, :the coil-,andoore structure C and the signal generating networks v652, 64 of Fig. 2 could be vplaced in a pilotless aircraft, together with a radio transmitter, while the remaining portion of the apparatus could be connected to a radio receiver located vat the base for the aircraft, and a vcontinuous indication of the aircraft bearing conveniently obtained. Similarly, the coil-and-core structure C and signal generating lnetworks 62, 64 could :be placed in a non-:magnetic buoy located in the entrance of a harbor, and the indicatorlu located on shore, with a radio link connecting the two, so that any distortion of the earths field due to ships or submarines entering the harbor would be indicated immediately at the shore stations.

Since these and other modifications vcould be made in the apparatus shown and described, .all within the scope and spirit of the invention, the foregoing is to be construed as illustrative, and not in a limiting sense.

What is Vclaimed is:

l. A magnetometer comprising, in combination, a pair of variably permeable members xed at right angles to each other, a pair of inductance coils wound one on each of said core members, signal generating networks coupled to each of said coils for generating signals at frequencies which are functions of theeffective -inductances of said coils, magnetic-flux-producing means .for establishing aconstant unidirectional magnetic flux in-each of said core members, and ymeans for measuring and indicating concurrently the direction of the vector sum of said eiective inductances as a function of the frequencies of the signals generated in each of said signal generating networks, said measuring and indicating means comprising a discriminator network coupled to each of said signal generating networks for producing D. C. voltages of relative magnitudes and polarities determined by the frequencies generated in said signal generating networks, and an indicator coupled to said discriminator networks for producing a composite indication of the voltages produced by said discriminator networks, said ux-producing means comprising a source of direct current connected to each of said coils for establishing magnetic-field-producing currents in said coils.

2. A magnetometer comprising, in combination, a pair of variably permeable members xed at right angles to each other, a pair of inductance coils wound one on each of said core members, signal generating networks coupled to each of said coils for generating signals at frequencies which are functions of the effective inductances of said coils, magnetic flux-producing means for establishing a constant unidirectional magnetic ux in each of sai-d core members, discriminator networks coupled to each of said signal generating networks for producing D. C. voltages of relative magnitudes and polarities determined by the frequencies of the signals generated in said signal generating networks, a cathode ray tube indicator, and means coupling said cathode ray tube to said discriminators for deflecting the cathode ray beam in said tube in accordance with the voltages produced by said discriminator networksI said last-mentioned means comprising a pair of similar circuits each having a separate amplier associated with each discriminator network, said amplifiers having at least a cathode. a grid, and an anode, means to apply said D. C. voltages of its associated discriminator network to said grid, a load resistor and a voltage divider in the anode cathode circuit of said amplifier, a pair of thyratrons each having at least an anode, a cathode and a grid, a first and a second resistor, the anode of said amplifier being connected through said rst resistor to the anode of one thyratron, to the cathode of the other thyratron and to said cathode ray tube indicator, an intermediate point on said voltage divider being connected through said second resistor to the cathode of said one thyratron, to the anode of said other thyratron, and to said cathode ray tube indicator, a capacitor across the anode and cathode of said one thyratron, and means to apply positive pulses to the grids of said thyratrons to cause conduction therethrough.

LESLIE L. BURNS, J R.

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